What are the "-" and "j" of "-9.96587885e+03j" with the number 1.54991569e+03-9.96587885e+03j?

This is a complex number, in the form of a+bj, where j is an imaginary unit. 1.54991569e+03-9.96587885e+03j is a complex number with a real part of 1.54991569e+03 and an empty part of -9.96587885e+03.

The imaginary units used in high school mathematics in Japan are often written in i, but in some fields, j is used. Python is supposed to treat literal in the form of a+bj as complex numbers.

https://docs.python.org/ja/3/library/stdtypes.html#typesnumeric

If you add 'j' or 'J' to the numeric literal, you can give an imaginary number (a complex number with zero real parts) and add an integer or floating point number to it to obtain a complex number with real and imaginary parts.

The scipy.ftpack.ft document also states that this function returns ndarray as a complex number element.

https://docs.scipy.org/doc/scipy/reference/generated/scipy.fftpack.fft.html

Returns z:complex ndarray